1. Field of the Invention
The invention relates to the supply of oxygen to a wastewater treatment system. More particularly, it relates to the supply of oxygen to such a treatment system from a pressure swing adsorption system under variable demand conditions.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes and systems are well known in the art for achieving desirable separation and purification of at least one gas component from a feed gas mixture of said component and at least one selectively adsorbable component. The selectively adsorbable component is adsorbed at a higher adsorption pressure after which the pressure is reduced to desorb said component from the adsorbent.
One use of the enriched oxygen product obtained in such PSA processes and systems is to supply suitable oxygen feed gas to a secondary wastewater treatment system. Such a system generally provides for the treating of water containing biologically oxidizable material with a gas containing at least about 50% oxygen by volume. Typically, such systems have been designed for the use of enriched oxygen product having a purity of about 90%, and PSA systems have been designed to supply enriched oxygen at this purity level to the wastewater treatment system.
Wastewater treatment systems of the type referred to above have an inherently variable demand for enriched oxygen product from the PSA system. Thus, such treatment systems are commonly designed for full capacity conditions that may not be fully achieved until the latter portions of the operating history of the facility in which said treatment systems are located. In addition, it is common for such wastewater treatment systems to exhibit variable load patterns as a function of daily or seasonable variations in water consumption. Accordingly, it is usually necessary for such wastewater treatment systems to be designed for maximum operating capacities that are expected to be fully utilized only during a portion of the expected operational life of the systems. In a similar manner, the pressure swing adsorption systems employed to supply enriched oxygen product to such variable demand wastewater treatment systems must likewise be designed for maximum production capabilities that are expected to be required only during a portion of the total operational history of a wastewater treatment facility. For a considerable portion of the operating time, therefore, the pressure swing adsorption systems in such facilities will be operated at lower than maximum capacity. It is highly desirable in the art, under such circumstances, to ensure that the PSA system employed to supply enriched oxygen to a variable demand wastewater treatment system can be operated in an efficient manner under reduced demand, i.e. less than full capacity, conditions.
During periods of reduced demand, a PSA-oxygen system. might ordinarily be operated at a correspondingly lower product/feed ratio that pertains under said full capacity or design conditions. Under such conditions, however, less enriched oxygen product is produced, and the recovery of oxygen product is reduced. While the purity of the oxygen recovered is higher than design purity under such lower product/feed conditions, the performance characteristics of the overall PSA-wastewater treatment system is diminished since such higher purity oxygen is not required, particularly under conditions of reduced, i.e. less than design, oxygen demand. Accordingly, PSA-oxygen systems having more efficient turndown capabilities were desired in the art, leading to the improved PSA control technique of the Pietruszewski patent, U.S. Pat. No. 4,140,495, wherein the product/feed ratio is maintained at the design level under reduced demand conditions, rather than being reduced as in the previous approach referred to above. The technique of the patent results in the production of oxygen product during periods of reduced demand over the course of a longer PSA processing cycle than would be used under maximum design conditions. As a result, the unnecessarily higher oxygen purity levels previously obtained are avoided, and a constant oxygen product purity is obtained.
For secondary wastewater treatment systems of the type herein of interest, the utilization of the oxygen supplied by the PSA system is a function of the oxygen feed gas purity. Those skilled in the art will appreciate that, for a given oxygen demand, i.e. a fixed requirement of oxygen, the utilization of available oxygen in the feed gas to the wastewater treatment system will increase as the purity of the feed gas increases. An increase in purity results in a higher dissolution driving force and causes additional oxygen to be dissolved in the wastewater being treated. It is, of course, desirable for wastewater treatment systems to have a high oxygen utilization, which is the utilization of a substantial portion of the oxygen supplied in the feed gas for treatment of the wastewater rather than being vented from the wastewater treatment facility as part of the waste gas from said facility. Typically, wastewater treatment systems have been designed to utilize about 90% of the oxygen feed to said systems when operated at design capacity, and PSA systems are typically designed to supply enriched oxygen feed gas at about 90% by volume purity to said wastewater treatment systems.
While the PSA control technique of U.S. Pat. No. 4,140,495 represents a substantial improvement over the prior practice, the supply of oxygen to the wastewater treatment system at a constant, predetermined design purity level is found not to be necessary under reduced demand conditions. For example, although dissolved oxygen levels of about 6 mg/liter are considered satisfactory for particular wastewater treatment operations, it has been found not unusual, at reduced demand conditions, for the dissolved oxygen levels to reach 10 mg/liter or more so as to result in excessive dissolution of oxygen in the wastewater. In this regard, it should be noted that it is not readily possible to unload the liquid mixing and oxygen dissolution equipment associated with the wastewater treatment system so as to avoid such excessive dissolution while nevertheless maintaining the biomass in suspension during reduced demand conditions. In addition, the constant oxygen purity technique of the patent is found to result in a decreasing oxygen product recovery under decreasing oxygen demand conditions as contrasted with the recovery at maximum design demand. Such oxygen product recovery decrease, indicative of reduced PSA performance at reduced demand conditions, is believed to be related to a more diffuse mass transfer zone with the adsorption bed under low gas flow conditions as are applied during periods of reduced demand.
While the increased oxygen utilization and increased dissolved oxygen levels achieved during the practice of the technique disclosed in the patent are, of themselves, desirable, the benefits thereof are not such as to obviate the desire for further improvement in the PSA supply of oxygen to secondary wastewater treatment systems. In particular, it is desirable to achieve improved PSA performance, i.e. higher oxygen recoveries, than has heretofore been possible in the art. In addition, it is always desirable to achieve swings in the power consumption for applying oxygen to the wastewater treatment system.
It is an object of the invention, therefore, to provide an improved PSA process for the production of oxygen for delivery to variable oxygen demand wastewater treatment systems.
It is another object of the invention to provide a process for enhancing product recovery in such PSA oxygen operations.
It is a further object of the invention to provide PSA oxygen process having reduced power requirements in conjunction with the supply of oxygen to variable oxygen demand wastewater treatment systems.